Rendering with FFmpeg vs PNG formats
FFmpeg is not a format.
It is a suite of libraries and programs used to create files using different containers and codecs, used mostly for video files.

PNG is a file format in which you can save images.

Which one to use depends on your needs.

Usually when creating animations or video it is recommended to render as an image sequence and encode to video later. That way, if the render stops or fails, you don't lose all of your work and you can try a different encoding and format until you find the one that works for you.